Add empty alt tag to non-responsive images

[bryophyta]

Note This hasn't been tested yet; pending getting the right container on CODE fronts

• Cards which have images that don't have a width property are rendered using a fallback image tag, which didn't have an alt attribute.
• An example is the Other sports container on uk/sports.
• This adds an empty alt attribute to the fallback image tag.
• The motive here is to improve accessibility. Having an empty tag here seems justified because in the cases we're aware of the image is basically decorative. Having an empty alt tag will mean that screenreaders don't try to read out the image url.

Does this change need to be reproduced in dotcom-rendering ?

• [x] No
• [ ] Yes (please indicate your plans for DCR Implementation)

Tested

• [ ] Locally
• [ ] On CODE (optional)